In consideration of influences on the environment, there is an increased need for removing particulate matter and harmful substances in exhaust gas from an internal combustion engine, a boiler, and the like. In particular, regulations regarding removal of particulate matter (hereinafter referred to from time to time as “PM”) discharged from a diesel engine tends to be strengthened on a global basis, and use of a honeycomb filter is attracting attention as a trapping filter (hereinafter referred to from time to time as a “DPF”) for removing PM, and various kinds of systems have been proposed. In the aforementioned DPF, generally a plurality of cells functioning as fluid passages and each having a quadrangular cross-sectional shape are separated and formed by porous partition walls, and the cells are alternately plugged in such a manner that the porous partition walls constituting the cells function as a filter. Here, the cross-sectional shape means a shape of a cross section cut along a plane perpendicular to the longitudinal direction of the cells.
The DPF allows the exhaust gas or the like containing particulate matter to flow therein from one end portion side and, after the particulate matter is filtrated by the partition walls, discharges purified exhaust gas from the other end portion side. The DPF has a problem of deposition of the particulate matter contained in the exhaust gas in the one end portion (end portion on the exhaust gas inflow side) in accordance with inflow of the exhaust gas to clog cells. This is a phenomenon easily caused in the case that a large amount of particulate matter is contained and in cold climates. Such clogging of cells causes rapid increase in pressure loss in the DPF. In order to inhibit such clogging of cells, there has been proposed a structure where a cross-sectional area of the cells each having an open end portion on the aforementioned gas inflow side (inflow side cell) is different from that of the cells each having an open end portion on the aforementioned other side (outflow side cell). Here, the cross-sectional area means an area of a cross section cut along a plane perpendicular to the longitudinal direction of the cells.
However, when the cross-sectional area of the inflow side cells is made different from that of the outflow side cells of the honeycomb filter having the cells having a quadrangular cross-sectional shape, the thickness of the partition walls forming the cells becomes thin in a part of a portion where the partition walls cross each other (hereinafter referred to from time to time as an “intersection portion”) to decrease strength. Therefore, when PM is combusted and removed by performing post injection when PM has deposited in the DPF, thermal stress concentrates in a part of the thin intersection portion to easily cause breakage. Here, the portion where the partition wall cross each other means a portion belonging to both the partition walls crossing each other in a cross section cut along a plane perpendicular to the longitudinal direction of the honeycomb filter. For example, in the aforementioned cross section, when the partition walls linearly extending and having the same thickness cross each other, the intersection portion means the square range where the partition walls cross each other.
In addition, there has been proposed a honeycomb filter having the inflow side cells having a larger cross-sectional area and outflow side cells having a smaller cross-sectional area, where the each of cells having a larger cross-sectional area has an octagonal cross-sectional shape obtained by linearly cutting the corners of a square (see, e.g., Patent Document 1).
By the octagonal cross-sectional shape of the cells, the partially thin state of the intersection portion is slightly reduced. However, the problem of low strength against thermal stress still remains. Therefore, there is desired a honeycomb filter where both the problem of clogging of the cells and the problem of low strength are solved.
Patent Document 1: French Patent Application Laid-Open No. 2789327 Specification